Major advances in molecular biology, cellular biology, and genomics have substantially improved our understanding of cancer. Now, these advances are being translated into therapy. Targeted therapy directed at specific molecular alterations is already creating a shift in the treatment of cancer patients. Glioblastoma multiforme (GBM), the most common brain cancer of adults, is highly suited for this new approach. GBM is among the most aggressive and deadly of neoplasms. Despite decades of aggressive surgical treatment, chemotherapy, radiotherapy, and extensive basic science and clinical research focused on combating this disease, the prognosis remains virtually unchanged, with survival rates still measured in months. The current genetic understanding of GBM has led to the identification of crucial intracellular molecules and their associated signaling pathways as potential therapeutic targets. Multiple genes are being identified as critical to the development of GBM and other similar tumors, such as gliomas and other astrocytomas.
Targets Involved in GBM
GBM, like most malignant tumors, exhibits multiple genetic abnormalities, including aberrant activation of intracellular signal-transduction pathways that regulate processes such as proliferation, angiogenesis, and apoptosis. For example, vascular endothelial growth factor (VEGF) is needed to promote tumorigenesis and angiogenesis in GBM (7). In addition, over 40% of all GBMs exhibit amplification, mutation, or rearrangement of the epidermal growth factor receptor (EGFR) (2). As is common in malignant neoplasms, EGFR overexpression leads to aberrant activation of crucial downstream targets, including the PI3K/AKT signaling pathway, which mediates cell survival, and the mitogenic RAS/MAPK cascade. Recent studies implicate a mutant variant of EGFR, EGFRvIII, in the activation of other receptor tyrosine kinases (RTKs), such as MET, thereby providing multiple inputs to these downstream signaling pathways for cellular proliferation and evasion of apoptosis. This redundant signaling could partially account for the modest response of GBMs to RTK inhibitors, such as erlotinib and gefitinib. In addition, GBM is frequently associated with elevated levels of O6-methylguanine-DNA-methlytransferase (AGT), a DNA repair enzyme that enhances neoplastic resistance to chemotherapeutics such as Temozolomide (TMZ).
Matrix metalloproteinases (MMPs) enhance tumor cell invasion by degrading extracellular matrix proteins, by activating signal transduction cascades that promote motility, and by solubilizing extracellular matrix-bound growth factors. MMP-9 and MMP-2 promote GBM invasion in vitro and in xenograft models, and their inhibition dramatically reduces the invasive phenotype (1). The serine/threonine kinase Raf-1 is involved in the regulation of tumor cell survival, proliferation and metastasis formation, and has therefore emerged as a promising target for cancer therapy. Raf-1 silencing appears as a potential therapeutic strategy to inhibit brain tumor angiogenesis and thereby outgrowth of highly vascularized glioblastoma multiforme (28). RNA interference targeting TGF-beta 1, 2 results in a glioma cell phenotype that is more sensitive to immune cell lysis and less motile in vitro and nontumorigenic in nude mice (29). The mammalian target of rapamycin (mTOR) activity is required for the survival of some cells within these GBMs, and mTOR appears required for the maintenance of astrocytic character in the surviving cells (30). A small molecule inhibitor of Cox-2 enhanced glioblastoma radiosensitivity, reduced tumor cell viability and prolonged survival of implanted glioblastoma mice by inhibition of tumor angiogenesis and causing extensive tumor necrosis (31).
RNA Interference (RNAi) and Small Interfering RNA (siRNA)
RNA interference (RNAi) is a sequence-specific RNA degradation process that provides a relatively easy and direct way to knock down, or silence, theoretically any gene. In naturally occurring RNAi, a double-stranded RNA (dsRNA) is cleaved by an RNase III/helicase protein, Dicer, into small interfering RNA (siRNA) molecules, a dsRNA of 19-27 nucleotides (nt) with 2-nt overhangs at the 3′ ends. These siRNAs are incorporated into a multicomponent-ribonuclease called RNA-induced silencing complex (RISC). One strand of siRNA remains associated with RISC and guides the complex toward a cognate RNA that has sequence complementary to the guider ss-siRNA in RISC. This siRNA-directed endonuclease digests the RNA, thereby inactivating it. Recent studies have revealed that chemically synthesized 21-27-nt siRNAs exhibit RNAi effects in mammalian cells, and the thermodynamic stability of siRNA hybridization (at terminals or in the middle) plays a central role in determining the molecule's function. These and other characteristics of RISC, siRNA molecules, and RNAi have been described.
Applications of RNAi in mammalian cells in the laboratory or, potentially, in therapeutic settings, use either chemically synthesized siRNAs or endogenously expressed molecules. The endogenous siRNA is first expressed as small hairpin RNAs (shRNAs) by an expression vector (plasmid or virus vector) and is then processed by Dicer into siRNAs. It is thought that siRNAs hold great promise to be therapeutics for human diseases, especially those caused by viral infections. Certain siRNA therapeutics are described in PCT application PCT/US2005/003858 for “Compositions and Methods for Combination RNAi Therapeutics”, which is incorporated herein by reference in its entirety.
Importantly, it is presently not possible to predict with any degree of confidence which of many possible candidate siRNA sequences potentially targeting a genomic sequence (e.g., oligonucleotides of about 16-30 base pairs) will in fact exhibit effective siRNA activity. Instead, individual, specific candidate siRNA polynucleotide or oligonucleotide sequences must be generated and tested to determine whether the intended interference with expression of a targeted gene has occurred. Accordingly, no routine method exists for designing an siRNA polynucleotide that is, with certainty, capable of specifically altering the expression of a given mRNA.